Angle modulated wave detector



Patented Oct. 1, 1 946 Y GeorgelClifford' Sziklai, Princeton, N, J., assigner to Radio Corporation ofA America, a corporation ofY Delaware l; Application May 20, 1944, Serial No. 536,457

My ,present invention relates generally to detectors of angle modulated carrier waves, and more particularly to novel 'circuits for detecting frequency modulated carrier waves.

is well known, in the absence of special precautions .the conventional frequency modulation (FM) detector not only has an output dependent on frequency variation, but also on intensity variation of the signal. Even in the case of a balanced FM detector circuit, :which has zero output at carrier frequency, crossemoclulation effects are obtained if the carrier. intensity changes during production of output energy in response to frequency deviation', i; e. simultaneously with reception of frequency-modulated signals. `In the past an amplitudeflimiter stagev has' been used prior to the FM detector to vminimize such carrier intensity changes.v Limiters are, in general,

10 Claims. (Cl. Z50-27) 'Cil v 2 Another object of my invention 'is to provide deflection. electrodes in a beam tube which are additionally adapted to -function as anodesof diode rectifiers.

Another object of this invention is lto provide a :beam tube detector for FM waves, wherein frequency variable waves causeV relatively different emission variations from'a pairof secondary emission surfaces thereby to. provide a' resultant useful output voltage.' U Still another object Yof this invention is to im prove reception of angle modulated carrier waves by producing variable secondary electron emission from an output electrode in response to phase departures of a pair of signal voltages from a .normal phase relation, variations in amplitude clipper tubes which require a minimum signal to operate, and thisA minimum signal (threshold improvement level) determinesthe sensitivity of the detector and of the receiver.` While a limiter normally has a high gain when it is not clipping, va comparatively large p-ortion ofthe gain is diverted intoV amplifying carrier harmonics e during actual limiting, For v'this reason the=sta bilityv of the preamplifier is'diminished for the 'l weak signal; or no+signal',stat e, and the receiver generally requires a noisemuting circuit to sup" press: reproduction of inter-'station'noises ywhich appear .during the normal tuning-process.

' Itis Aan important object of myinvention 'to provide a detector for FMfwaves which is substantially non-responsive to carrier intensity variation withoutrthe use of aspecial `limiterstage priorto thek FM detector. y, f'-

In the conventional andprior FM of the type employing` opposed rectiers, usually one electron emitter .cannot be Aconnected to' ground, and, therefore,za tube with indirectly heated cathode must'be used thereby 'requiring increased lamentpower. This latter fact is par--4 ticularly objectionable inbattery-operatedre'- ceivers.` Another'importan-t, object of my invention, thereforeris to'improve the construction and gain of FM detector tubes byemploying a beam `type of ,tube` permitting the use of a grounded primary electron emitter.` 2 l It is a further object 'of my presentinvention tron beam ofthe-tube so as to minimize the` effects of intensity variations of the received en-y ergyfon theA output current ofthe tube.

,-fdetectors of the signal voltages being Vautomatically compensated for by Ycontrolv of said Yvariable emiss1on.'v

' 'Still other objects of my invention are to'improve vgenerally the eliiciency and stability of FM receivers,and vto provide an amplitude limiter-Y free FM receiver system which is economical to manufacture and assemble.

The novel 'features which I believe to be characteristic ofmy invention are set forth with particularity inthe appended claims; the invention" itself,`however, las to both its organization and methodof operation will best be understood by reference tothe following description, taken in connectionwith theI drawing, in which I have indicated `diagrammatically several circuit organiza'tions' whereby my invention may be carried into` effect. Y

Inthe drawing: 1

Fig. 1 showsthe electrodes of a beam tube of the type employed Vin my invention,

Fig.2 shows FM detector circuits employing the Fig. 2a shows diierent positions of the electronV v wherein likereference numerals in the different figures designate similar circuit, elements, there is'shown in Fig. 1 a tube envelope I of any de-l sired conflguration.- The electron emitter of the tube is an indirectly heated cathode I' which is laterally arranged relative to the axis of the tube. The emitter I may be of the filament type, if desired. The emitter l provides a beam or sheet of electrons S which passes through a narrow space or slit between the forward wings or flanges 1 andv 8,of respective deflection plates 2 and 3.

The plates' 2 and 3 are spaced in parallel rela-r trol grids 4 and 5 and falls on target plate 6.

The latter, as shown, is provided with a central triangular area 6.

The target 6 is of conducting material, and one portion of its inner face is a high secondary electron emission surface, i. e. it emits a copious flow of secondary electrons upon bombardment by the beam of primary electrons S. However, the dark.

area 5 of the target 6 has a relatively low secondary emission characteristic, so that upon bombardment of area by the beam of electrons S relatively less secondary electrons are emitted therefrom. The area, 6' may be provided by a coating of carbon for poor secondary electron emission, while the high secondary electron emission may be provided by a caesium oxide coating. Further, the target 5 could be madey of silvermagnesium alloys, or caesium on silver. InA either of the latter casesthere would be formed on the target the low emission area 6. If desired, the area 5 of the target may be the smaller portion of theentirearea of target 5.

When theV beam S is deflected in such. a way that the major portion ofthe electrons thereof land on the high secondary emission surface of target 6,.a large number of negative secondary electrons leave the latter. Hence, the target assumes a relatively positive potential. On the other hand, when the beam is deflected so that the major portion of the electrons hits the low emission surface 5', less electrons leave the target 6 vthan are collected. Hence, the target becomes relatively negative in potential. The tube shown in Fig. l, therefore, provides a device for varying theeffe'ctive potential of an electrode by sweeping a beam of electrons across secondary emission surfaces of different emssivity in accordance with the relative potentials of a pair of beam control electrodes.

In Fig. 2 I have shownl tube I connected in an FM receiver to function as an FM detector. The FM receiver is assumed to be a superheterodyne receiver employed in the 42-50 megacycle (mc.) band, the presently-assigned FM broadcast band. In that band each carrier frequency is deviated in accordance with modulation signals. The extent of frequency deviation is a function of modulation signal amplitude, while the rate of deviation is dependent upon the modulation frequencies per se. The permissible frequency deviation or swing, in accordance with present broadcast transmitting standards, is a maximum of 75 kilocycles (kc.) to each side of the carrier frequency. The invention is not restricted to the FM frequency range of 42-50 mc., nor to FM wave reception, norto the specific over-al1 deviation range of 150 kc. The term angle modulated carrier wave used in this specification includesv phase modulated (PM) carrier waves or FM carrier waves, or hybrid modulations of PM and FM possessing characteristics of each.

The tube V1 is an amplifier preceding the usual discriminator network feeding the tube I. Amplifier V1 may be anon-limiting intermediate frequency (I. F.) amplifier whose input electrodes are coupled to a prior I. F. amplifier, or to a converter. The networks which precede I. F. amplifier tube V1 are of suitable and usual construction. They may comprise a signal collector and cally represented in Fig. 2)

one or more selective high frequency amplifiers followed, if the receiving circuits are of the superheterodyne type, by a converter which functions to produce the intermediate frequency (I. F.) signal energy.

The I. F. signal energy may be of a mean frequency chosen from a range of 2 to l5 mc., for example 4.3 mc. My invention is not limited to thespecic I. F. band, as values' up to 100 mc. can be used. The amplier V1 ampliiies the I. F. signal energy at the selected 4.3 mc. value. In the plate circuit of amplifier V1 there is arranged a resonant circuit Ci-Li tuned to the operating LF.. value.A The low potential side of circuit `C1-Li isv connected to the B-iterminal of the direct current energizing source through resistor 9, while condenser I Il bypasses all I. F. currents to ground. Resonant circuit Lz-Cz is tuned to the operating I. F. value. Coils L1 and L2 are magnetically coupled, while direct current blocking condenser: Ca'connects-'the midpointof coil Lz to the high alternating potential side of circuit C1-Li. The network Ci--Li and Cz-'Lz and its associated elementsprovide a, form of frequency discriminator disclosed and .claimed by S, W. Seeley ink his U. S. Patent No. 2,121,103, granted June. .21, 1938. My invention is not restricted, however, to this specific form of discriminator, as any other form ofsuitable frequencydiscrirninator circuit may be usedv priorto the tube I.

The outputterminalsof circuit Cz-Lz are connected to-the controlelectrodes of tube I. Electron emitter I of tube Iv (the latter'is schematiis connected to ground, and also tothe junction of resistors Ri and R2. The latter two elements are connected in series across' the tuned circuit C2-L2. The directv current blocking condenser I'I connects one side of secondary circuit Cz-Lz `to the upper end of resistor Ri, whereas the blocking'condenser I2 connects the opposite sideof circuit Cz-Lz to the lower end'of resistor'Rz.

Deflection plate 2 is connected to resistor Ri, and deflection plate 3` is vconnected to resistor R2. It will'be seen that each of diodes I- and I'--8.is connected in shunt; with a respective one of resistors R1 and R2, the junction of the latter being at ground potential. The grid 5 is establishedat a suitable positive potential B|+, the voltage supply lead 'being properly bypassed to ground for I. F. currents by condenser I`3. The grid 5 acts, therefore, as a positive screen grid. Grid 4 isconnected by lead I'II to condenser C3 so that the I. F. voltage across primary circuit Cr-Li is applied `to grid-f4. A resistor R3 is connected from grid 4 to ground thereby to develop across the resistor, in a manner further described hereinafter, a recti'f-'ledvoltage which, after filtering at IA" inthe usual manner, is used for A. V. C. bias. The IA. V. C. (automatic volume control) bias may be applied by lead I 5 to the signal control grids of prior signal transmission tubes for the customary A. V. C. action.

The target or secondary emitter 6 may be connected directly to ground. However, it is preferred rather to connect targetf to a potential point which is positive relative to ground, but which is kept substantially less positive than the potential of screen 5,. In Figs. 2 and 3 the lower positive potential of velectrode 5 is denoted by one'plus sign, in place of the two plus signs of screen 5. This positive potential of target 5 will ermit the landing of an increased number of primary electrons.- and thereby augments the emission of secondary electrons. VWhile it is trueA that `the target 6 Vvvouldvassume apositive lpotential due tothe net loss of electrons: anyway, target voltages of higher gaingcan. 'be obtained if the target is `startedat a positive potential. The screen 5, of course, functions. as an electron collector, since it collects all secondaryelectrons emitted by the target 6. The target- 6 is connected to its B+ terminal iby resistor -I-6 `bypassed to ground by condenserl6 for I. :F."cu-rrents.

The voltage developed across resistor 1611s modulation signal voltage. y AIt corresponds to, land is representative of, the modulation vsignals-employed to modulate the carrier at .the FM tra-nsmitter. The modulation-.amplifier has 'its in.- put grid electrode coupled to the resistor l-by blocking condenser 16ste amplify Vthe modulation signal voltage across the latter. The resistor l'iii is a high impedance elementand provides high gain forI amplier V2. The ampliedmodulation signal voltage isV developed across the resistor l1 arranged in the plate circuit vof tube V2; `Bypassed cathode bias resistor I9 provides normal negative bias of amplifier V2. The amplified voltage, when of yaudio frequency, Vvvill be furtheramplied and finally reproduced in anyr desired .man-

ner.

There will now be explained the manner in which the present circuit'functions to derive `the modulationvoltage from the FM waves. v"lhe I. F. or other high frequency signal energy atv primary circuit C1-L1 isinduced inthe secondary circuit Cz--Lz by the magnetic coupling therebetween. -The induced voltage'undergoes a phase shift of substantially 90 when the I. F. energy mean frequency is equal to the predetermined resonant frequency of the coupled tuned circuits. The phase-shifted voltage isy applied vto plates 2 and 3 in push-pull relation dueto the midtap on coil L2. The primary voltage isalsoapplied to the midpoint of coil L2 through condenser Ca- This voltagev is applied inv parallel relation to plates 2 and 3, and suffers no phase Yshift :since blocking condenser Gais too largeiiicapacity value to cause a phase shift inthe voltage applied therethrough. l

Hence, at each .of deflection plates 2' and 3 there exists Ya resultant voltage which, is produced by the vectorial combination of one of. the parallel voltages and one of ithe phase-shiftedvoltages. The resultant voltages atA the deflection plates are in phase quadrature and. of equal meurnitude for the conditionofr resonance Vbetween the mean. frequency ofthe LF. energy.v and .the predetermined resonant .frequency ofthe discrim# inator circuit. Should the meanxfreduency of applied I. F.. energyr instantaneously depart from the resonance condition, the phase relations will "titled .voltages derived: from the respective `1resultant voltages. 'Thesefrectiiied voltages will be equal at resonance, :and be' unequal `in Vresponse toinequalitylof the resultant vector voltages at the ends.:of..Lz-Cz. The effect. of the rectied 'voltagesracross R1 and :Ra impressedon deflectionr'platesfZ and 3 respectively, on the sheet of electronsS .will beto-deviate :the latter relative tov itsnormal median position andr inv accordance Withf-frequency'modulation of the received signal.

Fis. 2a shows the manner in which the Ytarget is'swept by the beam or sheetxof electrons S. When the: denection plate. 2' is more negative than the Y. denection plate 35 .the electrims will be f denec'tecitowards .thepplate 3. The'dottedline aol" Fig. 12u:` shows'the vertical line ofebombardment of targeti.'6"at .the .instant Whendeflection plate 2 is ,most .negative Yrelative to deflection yplate 3. Sinoeffew secondary electrons are emitted from thev high'emission surface of the output electrode' or target 6 because' such surfaceis not impactedhy `the electronv rbeam, and the area, 6 struck 4bythe electron beam'iszof-low secondary electron emissvity, so that negative electrons collectv 'on'. the target7 the target 6 is Yat its least positive potential.

Whenthe plates 2 and 3 are instantaneously of' the same negativepotential, as vvhen'the.v cen.- ter frequency `'ofi applied signal energy vis euual tothe. resonant frequency ofthe discriminator circuit,l they sheetV ofeiectronsr Sl will be in.. the medianposition.I `The dotted linev bl in Eig. 2u represents vthe line V4of bombardment ofl target 6 inv lsuch case. In this caserthe: electron beam may be dividedsubstantially equally between. the high secondary emission .arearand' 'the low emission area: 6" of.' the target V6, and. the output electrode '6l-assumes fa median potential which is positive relativefto'lts'fbiasingpotential B|. The potential "of-output. electrode 6 under this condition is more-positive'than in the case Whenthe electrons fall at line' a.. If the. deection plate 3 should'lassumewits most .negative potential, for whichV the circuits are designed relative' to plate 2, thenithe dotted' line c in Figa@ Would representthe deflectionof electrons towards plate 2'. Here the electronibeam impacts.' substantially entirely on the high; secondary electron emission area of the target electrode 6.. and :the maximum numberiof negative recording electrons; leaves the target. iHencathe electrode -61- attains its most posi-tivefpotential which most closely approaches the potential ofscreen grid VV5. It will accordingly.' bek understood that v.the- 'potential 'of output electrode 6' varies. from armure positive to aless depart from the quadrature relation, andv the re.- l

sultant voltages` Will Abe unequal. It will, therefore,.be seen that the resultant voltages on diode anodes T and 8 and the respective deflection'.platesl 2 and 3 varyy in relative magnitude in response to the frequency deviationsof the appliedl.. F. energy.

The beam or sheet of electrons S. whose plane is normal to the plane of target 6r is shifted or deviated in response to the variations of the. niag-- nitudesv of the resultant: voltagesprovidedat the deflection platesA 2 and 3. It will be recalled that tube V1 is a non-limiting amplierpan'd appliesboth frequency and amplitude variations of the carrierto the circuit Cl-Lr. 'The resultant volt-` ages at each'end of La-Czare reetiiied byr respectivediodes lll-l and 1"-8. I Thus.. 4thereis developed; across each of. resistors'. "RL and Rz. rece positive value as theelectron beam seweepsfrom linee tol'ine a, and from' a less positive to a more positive value as 'the electron beam moves in Ytheropposite 'direct-ion. This develops a correspondingY potential' across high impedance I6, which is ampliiied with high gain'Y at modulation amplifier V2. 'The secondary emission is collected. by screen 5. When this emission is large the target 6 appears to' be connected toscreen 5 through `the resistance of the secondaryemission. .In other Words, the secondaryelectron stream .from target. 6 to collector 5 -iunctionstsomewhat in the manner of va connector of variable resistance. The. magnitude of the effective resistance depends on the position of'beam S-onthe target 6'. When the omission from`-,target 6 -to'screenw is high, then target .i isfeiectively closer' to B++ in potential.

When beam is at line b of Fig. 2a, the applied FM waves are at mean frequency. A

As heretofore indicated, the constants of the tube l and its circuits are preferably so adjusted that the sheet of electrons S is swept between lines c and a of Fig. 2 as limits. The purpose of this is to keep the beam from sweeping to the right of line a, and rendering the electrode B highly positive thereby producing a false impression that the beam is really at line c; Of course, cutting the target at line a will, also, prevent over-swinging of the beam. In order to eliminate the effect of any change in the amplitude of the carrier, the primary voltage at C1-L1 is applied through condenser C3 without phase shift. The electrode 4 acts in th'e manner of an anode of a diode rectifier consisting of electrodes l and 4. Resistor Rs is the load resistor, and direct current voltages thereacross are proportional to carrier amplitude variations. The direct current voltage across resistor Rs will bias control grid 4 negatively with respect to ground. This negative bias will affect the intensity of the beam current and the consequent secondary electron emission from target 6.

Accordingly, the potential variation of target 6 will be reduced when the carrier amplitude is increased, and the reverse will be true when the carrier amplitude is decreased. This will prevent any increase in the slope of the typical discriminator characteristic if carrier amplitude increases. By the proper adjustment of the magnitude of resistor R3 the gain control by virtue of the action of grid 4 or the intensity of the electron beam can fully compensate for the varying deiiection due to carrier amplitude changes, simultaneously with the reception of frequency modulated signals. In other words, the variablebias grid 4 functions to minimize the effect of carrier amplitude variation on the electron stream S which flows to the target 6. Amplitude variations which occur at times of no signal reception balance out by having equal effects on the potentials of the deflection plates. For these reasons the amplifier V1 may be a non-limiter device. Furthermore, as heretofore stated,'the voltage across resistor R3 may be used for biasing previous signal transmission tubes thereby functioning as a usual A. V. C. circuit.

In Fig. 3 I have shown a modified form of cir- I cuit, wherein the deflecting plates of tube l do not function as diode anodes. In this modifica-- tion special electrodes l and 3 are employed on either side of the cathode i', the electrodes 1 and 8 functioning as the anodes of diodes l-'I' and |-8. The deflection plates are divergent, and are indicated by numerals 2 and 3'. The resistors R1 and R2 in this case are connected as before between the anodes and the common cathodes of the respective diodes. The compensating grid 4 is located in this modification closely adjacent to emitter i in order to provide better control action. Furthermore, a beam-forming electrode 9 is utilized between grid 4 and th'e input end of the deflection plates to provide a welldeflned beam on the target 6.

In the modified embodiment of Fig. 3 the resultant' vector voltages at the opposite sides of secondary circuit Cz-Lz are applied to diodes I'-l' and I-8 for rectification. Ihe load resistors R1 and R2, respectively in circuit with the last mentioned diodes, develop across each of them the required direct current voltages for variable biasing of deiiection plates 2' and 3'. The filters -2l and ZIV-2| prevent any I. F.

voltages from affecting the deection plates. Since each of plates 2 and 3 are connected to the respective anode ends of resistors R1 and R2, they will function in the same manner as eX- plained in connection with' Fig. 2. The control grid 4, being adjacent the electron emission surface of cathode l', will exercise a rigid control over the primary electron emission. As in the case of Fig. 1, the primary circuit (C1-L1) Voltage is applied to grid 4, and the rectified voltage across adjustable resistor Rs is used to vary the intensity of the beam of electrons sweeping the target 6. The rectified voltage across resistor R3, after proper filtering, may be used for A. V. C. bias. The embodiment of Fig. 3 functions otherwise in the manner described in connection with the preferred circuit of Fig. 2.

The area E of target 6 need not be restricted to a triangular configuration. If the beam of electrons is sufficiently thick, the target could have one path of its active face provided with a carbon coating and the other half could be a caesium surface. In this way according to the area of the beam landing on either surface, the potential of target 6 would vary. It is preferred to employ a triangular low emission area 6', since it is a simple configuration and is particularly suitable for the practice of my invention.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made vwithout departing from the scope of my invention.

What I claim is:

l. In combination with a beam tube of the type having an output electrode provided with a secondary emission area of predetermined non-uniform configuration, means, adapted to be responsive to phase displacements of a pair of alternating current voltages from a predetermined normal phase relation, for sweeping the electron beam of said tube over said area thereby to vary the potential of said output electrode and means responsive to amplitude variations of said voltages for automatically controlling the intensity of said beam.

2. In an angle modulated carrier wave receiver, a demodulator of the beam tube type provided with an output electrode having at least two secondary electron emission areas of different predetermined non-uniform configurations and of different emissivities, one of said areas completely surrounding the other area means for translating received waves into a pair of modulated wave voltages of equal magnitude for the condition where the mean frequency of the received waves is equal to that of a predetermined reference frequency, means responsive to instantaneous inequalities of said pair of voltages for varably deflecting the electron beam of the demodulator tube in such a manner as to variably sweep the beam across said two areas, and means for deriving from said output electrode a voltage representative of the modulation of the received waves.

3. In an angle modulated carrier wave receiver, a demodulator of the beam tube type provided with an output electrode having at least two secondary electron emission areas of different non-uniform configurations and of different emissivities, means for translating received waves into a pair of modulated wave voltages of equal magnitude for the condition where the mean fre- 'giostra quency of the received waves is equal to' that of a predetermined reference frequency, means re- Y sponsive to instantaneous inequalities of said pair of voltages for variably deflecting the electron beam of the'demodulator tube in such a manner as to variably sweep the beam across said two areas, means for deriving from said output electrode a voltage representative of the modulation of the received waves, and means, responsive to carrier amplitude variation, .for controlling the intensity of said beam in a sense to render said output electrode voltage independent of said `carrier amplitude variation.

4. In combination with a beam Itube of the type havingy an output electrode` provided with a secondary emission area of predetermined nonuniform configuration, means, adapted to be responsive to phase displacements of a pair of alternating current voltages from a predetermined normal phase relation, for sweeping the electron beam of said tube over said area'thereby to vary the potential of said output electrode, and means,

responsive to amplitude variation of said alternating current voltages, for controlling the intensity of said beam in a sense to render .the output electrode potential independent of said amplitude variation. Y

5. In a receiver of frequency modulated carrier waves, a. discriminator network for deriving from received waves a pair of voltages whose relative magnitudes -are a function of frequency deviations of the mean frequency of the received waves with respect to a predetermined reference frequency, means providing a beam of electrons, an output electrode provided with a non-uniform secondary emission surface adapted to be traversed by said beam thereby to vary the effective potential of the output electrode, a pair of beam control elements, and means responsive Ito variations in the relative magnitudes of said pair of voltages, for differentially biasing said beam control elements to control the secondary emissionV V tube over said area thereby to vary the potential of said output electrode, and means responsive to the amplitude of at least one of said alternating current voltages for controlling the intensity of the electron beam.

'7. In an angle modulated carrier wave receiver,

a demodulator of the beam tube typeprovided with an output electrode having at least two sec- Y ondary electron emission areas of different emissivities and different non-uniform shapes, one of 10 said areas completely surrounding the other area, means for translating received waves into a pair vof modulated wave voltages of equal magnitude for the condition where the mean frequency of the received waves is equal to that of a predetermined reference frequency, and means responsive to instantaneous inequalities of said pair of voltages for variably deilecting the electron beam of the demodulator tube in such a manner as to variably sweep the beam across said two areas.

8. In an angle modulated carrier wave receiver, a Ademodulator of the beam type provided with an output electrode having at least two secondary electron emission areas of different predetermined non-uniform configurations `and of different emissivities, means for translating received waves into a pair of modulated wave voltages of equal magnitude for the condition where the mean frequency of the received waves is equal to that of a predetermined reference frequency, means kresponsive to instantaneous inequalities of said Y pair of voltages for variably deiiecting the electron beam of the demodulator tube in such a manner as to variably sweep the beam across said two areas; and means, responsive to carrier amplitude variation, for controlling the intensity of said beam in a sense to render output electrode f voltage independent of said carrier amplitude V ling the intensity of saidbeam in a sense to render the output electrode potential independent of said amplitude variation. Y

`10. In a receiver of frequency modulated carrierL waves, a discriminator network for deriving from .received waves a pair of voltages whose relative magnitudes are a function of frequency y deviations of the mean frequency of the received waves with respect to a predetermined reference frequency, means providing a beam of electrons, an output electrode provided with a nonuniform secondary emission surface adapted to be traversed by said beam thereby to vary the effective potential of the output electrode, and

`a pair of beam control elements, responsive to `-variations in the relative magnitudes of said Vpair of voltages, to control the secondary emission from said surface.

GEORGE CLIFFORD SZIKLAI. 

